1. Field of the Invention
The present invention relates to a wireless information processing system, a wireless information recording medium, a wireless information processing apparatus and a communication method for the wireless information processing system. The present invention relates in particular to a wireless information processing system which exchanges signals between a plurality of wireless information recording media and a wireless information processing apparatus through wireless communication and to a communication method for the wireless information processing system.
2. Description of Related Art
Conventionally, a wireless card system which includes a wireless information processing apparatus and a plurality of wireless information recording media such as wireless cards has been available. The wireless information processing apparatus has a card reader/writer and the like which exchanges signals with these wireless cards through wireless communications. In this wireless card system, a communication method called a multi-read method is established, in which the card reader/writer receives response signals from the plurality of wireless cards simultaneously. Recently, the various improvement and relation technology on the multi-read method has been proposed, for example, at Japanese patent Laid Open (Kokai) No. H10-222622.
The multi-read method (first related art) is characterized by providing each wireless card with a time difference for the transmission of the response signals in order to accurately receive the plurality of response signals simultaneously. In other words, the number of time intervals, which corresponds to the number of the cards, is set for a response time period required to receive the response signals from the plurality of wireless cards simultaneously, and each wireless card respectively sends the response signals with respect to the response time intervals allocated by the wireless cards. Owing to this, it is possible to avoid collisions between response signals and accurately receive the response signals. Thus, it is possible to reduce the communication time.
A specific explanation will now be given for a case where a wireless information processing apparatus transmits a start signal which requests a response for card addresses from a plurality of wireless cards located in a communicable area (communication area). Together with the start signal, the wireless information processing apparatus transmits, for example, the number of response time intervals, composed of the maximum number of wireless cards that can be located in the communication area simultaneously. Upon receipt of the start signal, each of the wireless cards generates a random number to select one of the response time intervals and transmits a response signal including the card address and the like to a card reader/writer at the selected response time interval. As a result, the probability that the wireless cards will transmit response signals at different response time intervals is increased, and response signal collisions will be avoided.
The communication method according to the first related art has the following advantages and disadvantages depending on the number of wireless cards. Once the wireless information processing apparatus has transmitted a start signal, the apparatus needs to wait for responses from a plurality of wireless cards for a certain response period thereafter. When the maximum number of wireless cards that can be present in a communication area at once is small, the number of response time intervals is reduced. Accordingly, a response time as a whole can be reduced. Therefore, for start signals sent simultaneously, the wireless information processing apparatus can efficiently and accurately receive response signals from multiple wireless cards.
However, when there are a large number of wireless cards with which to communicate, the wireless information processing apparatus must set many time intervals, and after a start signal has been transmitted, must wait for responses from the wireless cards for a long response time. When smaller time intervals are set in order to reduce the response time, the probability of problems such as collisions between response signals, and the accurate reception of response signals occurring, is increased.
As described above, there is an upper limit on the number of the wireless cards for which the communication method according to the first related art can function effectively. Depending on an increase/decrease in the number of the wireless cards, it is difficult to appropriately perform the wireless communications. A multi-read method has been required, in which the advantages of the communication method according to the first related art are maintained while the disadvantages thereof are overcome.